In many processes, such as distillation or direct contact cooling, it is advantageous to use structured packing to promote heat and mass transfer between liquid and vapor streams which flow counter-currently to each other inside a column. Structured packing offers the benefit of high efficiency for heat and mass transfer combined with low pressure drop, when it is compared with dumped or random packing or with distillation trays. The most commonly used structured packing consists of corrugated sheets of metal or plastics foil or corrugated mesh cloths stacked vertically. While there are many types of structured packing described in the prior art, some of which are discussed below, they have evolved from a variety of applications and most have not been developed or optimized specifically for cryogenic separation processes, such as those used for separating the components of air.
EP-A-0321163 discloses the use of structured packing in a cryogenic air separation unit.
U.S. Pat. No. 5,100,448 (Lockett et al.) discloses that geometrically similar packing but of differing specific area can be used in different sections to maintain a common column diameter. The problem with this approach is that adjusting the packing density can have a significant negative impact on installed cost since there are only two alternatives: 1) increasing the packing density which decreases capacity but results in a significant increase in the cost of packing due to the increase in the amount of surface area per volume of packing, or 2) decreasing the specific density which increases capacity but at the expense of increasing the HETP (discussed below). This reduction in performance increases the height of packing required for separation or, if the column height is fixed, decreases the overall column performance. Thus this practice does not make full and effective utilization of the column volume and height which, for air separation plants, is of paramount importance.
U.S. Pat. No. 5,132,056 (Lockett et al.) describes a structured packing with corrugations and surface texture and also discloses the use of edge modification to improve wetting, especially under turndown conditions.
U.S. Pat. No. 5,454,988 (Maeda) discloses the use of special fluting in a corrugated packing with no holes. The fluting generally runs in a horizontal direction and is more square-wave like than sine-wave like in cross-section. The fluting also has a meandering flow path for laterally spreading liquid.
In Ind. Eng. Chem. Res. 1994, 33, pp1208-1221 and Ind. Eng. Chem. Res. 1994, 33, pp1222-1230, the disclosures of both of which are incorporated herein by reference, there is a discussion of a model for a two-phase pressure drop in a packed column.
It is also well-known in the prior art that mesh type packing helps spread liquid efficiently and gives good mass transfer performance, but mesh type of packing is much more expensive than most of the foil type packing described above.